The excitement was at its peak when FC Copenhagen visited Brøndby Stadium on 24 October for another showdown, and the home team’s supporters cheered intensely on their heroes. But the excitement was not only high in the stands. Underneath them, engineers from the consulting company Rambøll walked around with their eyes turned upwards:
The match was the first in almost three years with full occupancy in the lower stands of Brøndby Stadium’s famous South Stand (“Sydsiden”), after measurements of the stands’ movements in November 2018 had shown that the so-called allowed bearing capacity of the up to 14-meter-long TT elements was utilized by 135 percent when the spectators in the South Stand jumped along to the fight songs.
“Our calculations showed that the elements could withstand a deflection of 31 mm when we took into account all safety features. Shortly after the match, we analyzed the results and saw a 45 mm deflection in one of the elements, and a slightly smaller deflection when the element swung up again. It was too large to recommend continuing to use the stands as they were,” Claus Pedersen says.
He is a senior chief consultant in Rambøll’s department for monitoring and analysis of existing structures, and together with his colleague, chief consultant Torben Bilgrav Bangsgaard, he immediately sent the report with the warning to Brøndby. The same afternoon, the two sat in a meeting with six of the football club’s high-level officials and stadium manager Tom Larsen.
“We got the report on Wednesday and had a home game again on Sunday. So we had to react quickly. None of us wanted to be responsible for letting the spectators enter the stand with that risk, so we made a quick decision,” Tom Larsen says.
Therefore, Brøndby had already for the subsequent match mounted seats on the South Stand, cordoned off the most heavily loaded areas and reduced the spectator capacity to 2250—less than half the number that was normally allowed in the stand.
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The South Stand consists of terraces without seats, so that the spectators can stand closer to each other and move more freely. During the construction in 1992, the lower stands were approved for 5200 spectators, which was later lowered to 4700 due to escape routes. But over the years, the celebrations in the stands have become more extensive.
“We played a match against Frankfurt 15 years ago, and the whole stand section that their fans were standing on rocked. In the years since, we have also seen cracks in the brackets that carry the stand elements. That is why we kept an eye on the cracks and have had advisors examine them on a regular basis,” Tom Larsen says.
In 2016, Brøndby approached Rambøll to get expert engineering help to measure the fluctuations in the stand at an FC Copenhagen match. The club wanted to film a video about the atmosphere in the South Stand, which ended up resembling an earthquake.
On that occasion, Ramboll’s department for monitoring and analysis of existing structures measured an acceleration of 1 g—which corresponds to the acceleration of gravity—in the middle of one of the South Stand decks. Converted to deflections, the stand swung about 25 mm, which did not give rise to an assessment that the jumping could threaten the structure.
“One should not be nervous about standing in the stands. It’s concrete, it contains steel, and it’s reasonably robust in response to large fluctuations,” Torben Bilgrav Bangsgaard explained at the time in the video that Brøndby subsequently published.
But in connection with the club’s periodic inspection of the stands in 2018, Rambøll was again hired to monitor the fluctuations during a match against FC Copenhagen. And this time, the measurements showed fluctuations of up to approx. 1.8 g. This corresponds to the stands bending 45 mm downwards about twice a second and thus, according to Rambøll’s safety assessment, this caused Brøndby to introduce restrictions on the use of the stand.
Rambøll was quickly tasked with calculating what it would cost to replace the stand elements, and they came to the conclusion that it would be DKK 38 million—including some improvements. The price was so high that the engineers started looking at what it would take to save the existing construction.
In the autumn of 2019, the engineers therefore again started measuring deflections and acceleration in the stands during the matches, while also closely studying the standards:
“The standards presuppose that the spectators are really good at jumping simultaneously—regardless of whether there are 2, 20 or 400 spectators next to each other. But all the videos we recorded at the games showed that there is a miscorrelation in people’s ability to jump simultaneously. In addition, the standards presuppose that a jump entails the same load impact no matter how fast one jumps. But you don’t jump as high at 3 Hz as at 2 Hz. There is simply no time for that,” Torben Bilgrav Bangsgaard says.
The pace of jumping at football matches is determined by the capos, who direct the fan drums and choose the fight songs. Therefore, the engineers also threw themselves into analyzing the rhythm of Brøndby classics such as “Stoltheden er kolossal” and “Aldrig alene”.
The conclusion of all the analyzes was that a number of conditions made the load lower than assumed in the standards: In practice, the spectators do not jump simultaneously, and they jump less high the faster the rhythm of jumping. But even if this is taken into account, it would still be necessary limit the fluctuations.
“The stand decks are not connected. They just lay on the raker beams below. If the 30–40 spectators standing on the individual deck jump simultaneously, there may be violent fluctuations, as we saw in 2018. Therefore, we decided to connect the stand decks. Then it would instead take 300–400 fans jumping simultaneously to cause large fluctuations of the whole stand,” Torben Bilgrav Bangsgaard explains.
Therefore, Rambøll proposed to bolt the stand decks on the South Stand together via a railing-like steel structure on the upper side and a steel structure on the underside in the places where a railing on the upper side would require a larger parts of the stand to be kept free as escape routes.
“It would have cost us 400–500 seats, and that is a lot of lost ticket revenue for us. So it was worth building the steel construction under the stands,” Tom Larsen explains.
On the underside of the central stand deck, 14-meter-long steel beams have been added in some places, which increased both its rigidity and load-bearing capacity. The combination of the railing connections and the longitudinal beams reduces the load on the existing TT elements so much that the load-bearing capacity is sufficient.
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However, despite the reinforcements, the stands cannot be approved with the methods from the current standards. Therefore, Rambøll suggested that the number of spectators should be gradually increased, and the movements of the stands should be monitored and measured until there was no longer any doubt about their durability.
Because this is a project in the highest consequence class, CC3 +, third-party controllers are involved in checking Ramboll’s calculations—a certified statistician and an expert in rhythmic live loads. Although the latter is an FC Copenhagen fan, the plan was approved, and in February 2021, the municipality granted the building permit.
In the months that followed, a total of about 30 tons of heavy steel structures were installed, but coronavirus restrictions meant that spectators were still not allowed to enter the stadium. At the end of April, however, the Danish Parliament allowed the audience to return, but for the finals on 24 May, the South Stand was still not at full capacity.
To speed up the approval process, Rambøll proposed to hold a big “jumping test” with several hundred fans in the stands.
“It would cost a bit more, but in terms of data quality, it would be as good as or better than measuring during a real match. For example, we would be able to investigate the effect of jumping with different frequencies from 17.7 to 3 Hz, which would not be possible during a real match,” Claus Pedersen says.
The municipality approved the plan, and for the club there was also a great financial gain in being able to let several thousands of fans into the stadium earlier than they could otherwise have done.
“We have lost about DKK 250 000–300 000 in ticket revenue per match, so we were interested in the idea,” Tom Larsen explains.
On June 11, 400 Brøndby fans assembled in the South Stand. Ramboll’s engineers had rigged the large monitoring equipment and measured, while ever larger groups of spectators were let in the stands, where they jumped and sang—first under the leadership of the capos, but later conducted by the engineers. The measurements showed that the reinforcements were more effective than calculated.
“We had calculated that the decks would have a deflection of up to 31 mm. To achieve a little better security, we had set our own limit at 90 percent, which is equivalent to 28 mm. But the deflection was only 19 mm,” Torben Bilgrav Bangsgaard says.
After measurements at three matches, which were held with seats on the South Stand, all measurement results were sent to the municipality in early October, and on 16 October, the municipality allowed the seats on the South Stand to be removed and the number of spectators to be increased to 4700.
On 24 October, Brøndby then defeated its arch-enemy FC Copenhagen with the fans singing, jumping and dancing in the South Stand. The engineers under the stand were also satisfied:
“We measured a deflection of 22 mm. That is 15–20 percent more than during the jumping test. But that was also to be expected because this time it was exclusively the most dedicated fans who are used to jumping simultaneously. And it was still far from our limit of 28 mm,” Torben Bilgrav Bangsgaard says.
Brøndby has now installed a permanent remote-controlled monitoring system, and the engineers believe that in the long run, it should be enough for them to monitor four matches a year. And maybe they will be paying a visit to other clubs too:
“I have received inquiries from clubs that are very interested in what we came up with here. I don’t know if there are other stadiums that are designed in the same way, but we would be happy for our experience to give rise to the practice of checking stands,” Tom Larsen says.
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